70 NOTES AND MEMORANDA. 



" Mr. Browning has described^ the plan that he proposes 

 for the measurement of the position of absorption-bands by 

 means of a bright line, seen by reflection from the surface of 

 the prism, moved backwards and forwards by a micrometric 

 screw with a graduated head. My objection to the origitial 

 construction was that the bright line was photogi'aphed ou a 

 small piece of glass, and the background was so far from 

 being black as to much impair the spectra of substances that 

 will not transmit a bright light. I suggested that in place 

 of this glass p^ate a small piece of tin-foil should be used, 

 having a verj Yiinute hole in it. This shows far brighter 

 than the line ii. the photograph, and the background is quite 

 black ; and thus the bright dot can easily be seen even when 

 in the brightest part of the spectrum, and there is no extra- 

 neous light to impair the faintest absorption-bands. The 

 only important objection to this method of measuring their 

 position is, that a very slight movement in the apparatus, 

 due to the loose fitting of movable parts, alters the readings, 

 and that the value of the measurements, as read off by the 

 micrometer, depends on so many variable particulars, that 

 nearly every instrument might have a different scale. The 

 chief objection to my interference scales" is the difficulty of 

 making all agree absolutely, but when accurately made they 

 have not the above-named disadvantages. I therefore still 

 adhere to that plan, but at the same time I have found the 

 bright dot arrangement very useful, not only as an indicatoi* 

 in showing spectra to others, but also as a fixed point in 

 comparing different spectra, or in counting the bands of the 

 interferencf scale. Possibly without such help some observers 

 might find this difficnlt, and would prefer in all cases to 

 measure the position of bands by means of the graduations 

 on the circular head of the micrometer, and therefore I was 

 anxious to devise a prism that would have a dispersive power 

 intermediate between the two extremes already mentioned, 

 and at the same time have the upper face inclined at an angle 

 of 45" to the axis, so that the bright-dot micrometer might be 

 employed conveniently. To accomplish this, Mr. Browning 

 made for me a prism composed of two rectangulars of crown 

 glass, one rectangular of very dense flint, and one of less 

 dense, cut at such an angle as to give direct vision. This 

 combination gives what I consider to be as good a medium 

 dispersion as could be Avished, and at the same time enables 

 us to measure the position of the bands with the bright-dot 



1 'Monilily Microscopical Journal,' vol. iii, p. 68. 

 ^ ' Proc. lioy. Soc.,' vol. xv, p. i'.il. 



